Generally, in power devices, in order to drive a three-phase AC motor and the like, inverters that convert a voltage from DC to AC are used. In the inverters, it is required to electrically insulate between a high voltage applied to the AC motor and a control unit, so that photo couplers have been ever used as insulation elements. However, in recent years, with the progress in downsizing and thinning of transformers, the photo couplers are substituted with pulse transformers and/or insulation elements using capacitors, that are superior to the photo couplers in reliability, power consumption, integration degree, transfer speed, etc. Meanwhile, for example, signal transmitting circuits that transmit transmission data through the insulation elements are used with the three-phase AC motors and the like, and thus they are required to suppress their erroneous outputs due to noise from the motors and the like.
Meanwhile, for example, in Japanese Patent Application Laid-open No. 2010-56593 (hereinafter, referred to as Patent Document 1), there is disclosed a low-power and fast transmission/reception technology of asynchronous induction coupling type in which, in response to each level change at the rising and falling times of transmission data, a transmitter feeds a positive or negative unipolar current signal in pulse form to a transmitter coil, and in association therewith, an induction voltage signal of a pair of consecutive before-after pulses (hereinafter, referred to as double pulses) having both positive and negative polarities is produced in an inductively-coupled receiver coil whereby the transmission data can be demodulated by receiving the induction voltage signal asynchronously.
According to the technology in Patent Document 1, the induction voltage signal of double pulses induced in the receiver coil is detected by a hysteresis comparator, and every time of that detection, the positive or negative unipolar pulse is outputted followed by this pulse output being inputted to a D flip-flop to thereby restore the transmission data; or instead, with respect to induction voltage signals of respective the first and the second pulses (hereinafter, each referred to as a single pulse) in the double pulses induced in the receiver coil, the first single pulse is not detected but the second single pulse is detected by a hysteresis comparator and is then inverted, to thereby restore the transmission data.